diff --git a/interfaces/cython/cantera/test/utilities.py b/interfaces/cython/cantera/test/utilities.py
index f507eb298..fa0d391fc 100644
--- a/interfaces/cython/cantera/test/utilities.py
+++ b/interfaces/cython/cantera/test/utilities.py
@@ -1,9 +1,10 @@
 import numpy as np
 import unittest
 
+
 class CanteraTest(unittest.TestCase):
     def assertNear(self, a, b, rtol=1e-8, atol=1e-12, msg=None):
-        cmp = 2 * abs(a - b)/(abs(a) + abs(b) + atol)
+        cmp = 2 * abs(a - b)/(abs(a) + abs(b) + 2 * atol / rtol)
         if cmp > rtol:
             message = ('AssertNear: %.14g - %.14g = %.14g\n' % (a, b, a-b) +
                        'Relative error of %10e exceeds rtol = %10e' % (cmp, rtol))